Added a test vector for Argon2i

Added a test vector for Argon2i

Libsodium's API doesn't let the user specify the `key` and `ad`
arguments.  An implementation that flips them by mistake would still
pass the test vectors.

So I added a test vector from the reference implementation (hard coded,
to avoid dragging the whole reference implementation with us).  With
that, we're sure `key` and `ad` are processed in the right order.

It wouldn't have affected security, but due diligence can't hurt.